This invention relates to a process for producing substantially anatase-free TiO.sub.2 in a vapor phase oxidation of TiCl.sub.4 in a plug flow reactor by addition of a silicon halide at a process temperature from about 1200.degree. C. to about 1600.degree. C. at one or more points downstream of where an oxygen-containing gas and TiCl.sub.4 are initially contacted.
The process for producing TiO.sub.2 pigment by reacting O.sub.2 -containing gas and TiCl.sub.4 at temperatures ranging from 900.degree. to 1600.degree. C. in a vapor phase is known. The resulting hot gaseous suspension of TiO.sub.2 particles and free chlorine are discharged from the reactor and must be quickly cooled below 600.degree. C. within about 5 to 100 seconds. This cooling is accomplished in a conduit, i.e., a flue so that undesired TiO.sub.2 particle size growth is prevented and particle agglomeration is minimized. Pigment product properties such as carbon black undertone (CBU) and gloss are a function of primary particle size and particle agglomeration, respectively. If high agglomeration of TiO.sub.2 results, the TiO.sub.2 must be milled or ground in an energy intensive, expensive process such as micronizing to reduce the size of agglomerates in order to achieve the desired pigment properties. Further, two crystal structures of TiO.sub.2 may form: rutilie and anatase. The rutile TiO.sub.2 is preferred for its higher durability and its higher refractive index, both of which are important performance characteristics. Anatase TiO.sub.2 is inherently less durable than rutile TiO.sub.2 and possesses a lower refractive index. The presence of the anatase phase will in particular adversely affect durability. Even low levels of anatase affects durability of a finished TiO.sub.2 pigment. It is well known in the prior art that addition of a silicon compound during the oxidation stage promotes anatase formation.
Particle size has been controlled previously by premixing volatile silicon compound and TiCl.sub.4 prior to reacting it with the oxygen-containing gas. For example, British Patent 689,123 discloses premixing volatile silicon and aluminum compounds with TiCl.sub.4, oxidizing and resulting in TiO.sub.2 pigments with greater than 90% rutile. Therein, the aluminum serves as a ruffle promoter, but also causes the formation of coarser particles and the silicon compound serves to decrease particle size.
U.S. Pat. No. 3,219,468 provides a process in a dynamic bed reactor and discloses separate addition of silicon halide from all other reactants. Therein, however, the temperature is raised to no more than 1200.degree. C. and less than 0.3% SiO.sub.2 is present. When higher concentrations of the silicon halide is used therein, more anatase is formed.
The need therefore exists for a process for preparing improved quality of TiO.sub.2 pigment with decreased particle size and decreased agglomeration thus reducing the need for grinding operations while concomitantly eliminating the formation of anatase without loss of durability of the TiO.sub.2 pigment. The present invention meets that need.